The points are valid.  I suppose I just expect that the tiny file will reside 
in memory and require little to serve it (beyond as you stated, initiating a 
session), resulting in little benefit.  Just seems like we're stating something 
that could/should be left to the implementer/admin at the site to implement as 
is proper for their environment.

I believe Daniel was waiting on comments from Leif (and Viktor?)about his 
proposed language changes.

--
Alex Brotman
Sr. Engineer, Anti-Abuse
Comcast


-----Original Message-----
From: Uta [mailto:[email protected]] On Behalf Of Viktor Dukhovni
Sent: Monday, October 09, 2017 3:55 PM
To: [email protected]
Subject: Re: [Uta] Updated MTA-STS & TLSRPT

On Mon, Oct 09, 2017 at 06:24:36PM +0000, Brotman, Alexander wrote:

> Do we think we really need to allow caching?  From what are we trying 
> to protect the backend systems?

NOT the backend systems, rather the rather more constrained reverse proxies of 
the customers.

Caching can of course be disabled by provider, but I think that's needless 
inconsiderate of the reverse proxies.

> Feels like it would be easier to disallow caching (or recommend 
> against).  I understand the sense that a smaller company could be 
> overwhelmed by an attacker or ill-configured legit sender, but I don t 
> know how much caching will really help there.  The caching server 
> would be similarly overwhelmed I would imagine, and be unable to serve 
> policies to any other requesting systems.

The defense is not against deliberate DDoS, rather it significally reduces 
proxy concurrency, by making it possible to serve content from a cache, rather 
than go upstream for each request.  This can significantly improve scalability 
at little cost.

All the provider has to do is delay the publication of the TXT record, until 
the corresponding new policy has been the only one available for > 60s.  This 
is a low cost to pay, and allows cooperative caching to be supported by 
providers that wish to do so.  I think that providers should support caching, 
but this specification will not require them to do so.

-- 
        Viktor.

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